In utero and early life experiences affect physiological development and can influence sensitivity to environmental factors throughout life. Data on early life risk factors, however, are typically not available to investigators who study associations between ambient air pollutant concentrations and pediatric asthma exacerbations. In the state of Georgia, birth records (1994-2007) have been linked with pediatric emergency department visits (2002-2007) by staff at the Georgia Department of Human Resources. This novel linkage between 1.8 million birth records and 5.5 million pediatric emergency department visits will be used to investigate whether children who were born premature or low birth weight are more sensitive to ambient air pollutant concentrations than their counterparts. If effect modification by gestational age or birth weight is observed, then these findings will help inform public health policy decisions, since protection of sensitive subgroups is central to EPA's mission as a regulatory agency. Further, within Metropolitan Atlanta, a research air quality monitoring station provides daily concentrations for a comprehensive suite of components of particulate matter less than 2.5 microns in diameter, a resource also not typically available to investigators. These measurements can be used to distinguish among the various compounds that comprise particulate matter less than 2.5 microns in diameter, and the proposed study will be the first to investigate associations between these compounds and pediatric emergency department visits in a birth cohort. Complex interactions between ambient air pollutants and airborne pollens on pediatric asthma exacerbations will be investigated using an innovative modeling approach based on two- dimensional loess smoothers in the framework of generalized additive models. Ambient air pollutants are known to cause asthma exacerbations in children. To inform public health policy decisions it is important to know whether certain subgroups of asthmatic children have increased susceptibility to air pollutants, and the proposed study will be the first to investigate whether children who were born premature or low birth weight have an increased susceptibility to air pollution.